Rapid Microbial Uptake and Mineralization of 14C-Labelled Cysteine and Methionine Along a Grassland Productivity Gradient

Cysteine (Cys) and methionine (Met) constitute important sources of directly available carbon (C), nitrogen (N), and sulphur (S) for both plant nutrition and microbial growth and are central to terrestrial S cycling. They are the only two S-containing amino acids that are incorporated into proteins. The factors controlling turnover rates of these two S-amino acids in soil, however, remain understudied. Due to their multi-nutrient composition (C, N and S) we hypothesized that intense competition would exist in the microbial community for these substrates. Further, we hypothesized that that this competition would be inversely related to plant productivity and thus C inputs to soil. To address this, we studied the short-term, concentration-dependent microbial uptake and mineralization of 14 C-labelled Cys and Met in five grassland soils taken from an altitude-driven primary productivity gradient. Substrate depletion from soil solution was measured over a 60 min period using a centrifugal drainage procedure and the subsequent evolution of 14 CO2 measured with NaOH traps over a 48 h-period. Our results revealed that 14 C-Cys and Met are directly and rapidly assimilated by soil microbes, with half-times ranging from 0.34 - 2.14 mins. This is an order of magnitude (or more) faster than when determined from measurement of 14 CO2 evolution. This considerable delay between microbial 14 C removal from soil solution and subsequent 14 CO2 evolution indicates that the degradation of Cys and Met in soil solution was mainly through biological processes. The use of different substrate concentrations suggested that when lower concentrations of 14 C-Cys and Met were added to soil (0.01 - 0.1 mM), soil microbial uptake occurred via a single carrier system, while a second transport system/mechanism became important when the substrates were added at higher concentrations (1 - 100 mM). Rates of substrate mineralization by soil microbes declined in less productive, nutrient limited grassland soils with lower levels of microbial biomass, suggesting that the turnover of organic N and S, and subsequent availability for plant uptake is likely to be controlled by the size and activity of the soil microbial community. We conclude that despite the relatively low concentration of Cys and Met in soil solution, these DON/DOS compounds could be of great significance in terrestrial nitrogen and sulphur cycling due to their rapid microbial turnover.